The present invention relates to a hands-free telephone and, more particularly, to a voice-switched type hands-free telephone which may be used for a mobile telephone system.
A hands-free telephone has a howling or singing problem. A voice-switched speakerphone is an answer to the singing problem. Reference is made to "Fundamental Considerations in the Design of a Voice-Switched Speakerphone," by A. Busala, THE BELL SYSTEM TECHNICAL JOURNAL, Volume XXXIX, No. 2, March, 1960. To avoid the singing, the voice-switched speakerphone, which will later be described referring to FIG. 1 of the accompanying drawings, includes a receive variable attenuator, or variolosser, inserted into a receive path and a transmit variable attenuator, or variolosser, inserted into a transmit path. The voice-switched speakerphone also includes an attenuation control circuit which controls the attenuation of the receive and transmit variolossers in response to the signal levels of the receive and transmit paths.
More specifically, when the output level of microphone is smaller than the input level of speaker, the attenuation control circuit causes the receive variolosser to decrease its attenuation while causing the transmit variolosser to increase its attenuation. When the output level of microphone is larger than the input level of speaker, the attenuation control circuit causes the receive variolosser to increase its attenuation while causing the transmit variolosser to decrease its attenuation. Thus, the voice-switched speakerphone can alleviate the singing during the conversation.
However, the voice-switched speakerphone still has the singing problem during a transient period from the origination of a call to the beginning of the conversation. This may be caused by unbalance of a hybrid transformer of a telephone exchanger. The unbalance increases coupling between a transmit signal and a receive signal. The increasing of the coupling induces the singing at the speakerphone.
In the mobile telephone system, a mobile base station to be connected to a mobile subscriber station is almost always changed when a call is placed from the mobile subscriber station. That is, a communications line to be connected to the mobile subscriber station is changed, resulting in changes in the coupling between the receive and transmit signals. This aggravates the singing problem during the transient prior mentioned above.